1. Field of the Invention
The present invention relates to a high-power and high speed modulation semiconductor laser element used as a transmission light source for wireless optical communication, and to an electronic device using the same.
2. Description of the Related Art
For a semiconductor laser element used as a transmission light source for wireless optical communication, a high-power laser element which enables high speed modulation and produces a perfectly circular, uniform radiation pattern is desirable. However, in a commonly-employed semiconductor laser element, when the output level of the laser light becomes high, an emission face is dissolved. (This phenomenon is referred to as “catastrophic optical damage (COD)”.) COD occurs as described below.
In the vicinity of the emission face, there is a surface level which does not contribute to the light emission. Because of this, when a current is injected into this surface level, carriers are converted to thermal energy without being consumed for light emission, and therefore, the temperature increases on the emission face. Such an increase in temperature further reduces the bandgap, such that light is absorbed by the emission face and the temperature increase is further accelerated. Thus, as the temperature increases in this positive-feedback fashion, COD occurs, i.e., the emission face of the laser light is dissolved.
For the purpose of preventing the occurrence of COD, in general, the density of light is decreased in the vicinity of the emission face, or the emission face and the vicinity thereof are made transparent to the emission wavelength so that the absorption of light is suppressed.
“Applied Physics Letters”, vol. 60, no. 6, pp. 668-670 (J. L. Mawst, et al) proposes a phase-locked array semiconductor laser element 50 as shown in FIG. 12. In this phase-locked array semiconductor laser element 50, a first lower cladding layer 52 and an active layer 53 are sequentially provided on a semiconductor substrate 51. On the active layer 53, a plurality of ridge portions 54 are provided with appropriate spaces therebetween. In each ridge portion 54, a second lower cladding layer 54a, a wave-guiding layer 54b, and a second upper cladding layer 54c are sequentially provided. In addition, on the active layer 53, a first upper cladding layer 55 is provided so as to cover the ridge portions 54.
The refractive index n2 of the active layer 53 is greater than the refractive index n1 of the first lower cladding layer 52, and is smaller than the refractive index n3 of the first upper cladding layer 55 (i.e., n1<n2<n3). The refractive index n5 of the wave-guiding layer 54b in each ridge portion 54 is greater than the refractive index n2 of the active layer 53, and is greater than the refractive index n4 of the second lower cladding layer 54a and smaller than the refractive index n6 of the second upper cladding layer 540 (i.e., n2<n5, n4<n5<n6).
In the phase-locked array semiconductor laser element 50 having such a structure, at each ridge portion 54, light generated in the active layer 53 is confined in and guided by the wave-guiding layer 54b. In areas between the ridge portions 54, light generated in the active layer 53 is confined in the active layer 53 itself and travels therethrough. According to such a structure, when all of the phases of guided laser light are synchronized, the oscillation threshold current is a minimum, and the phase-locked array semiconductor laser element 50 emits light.
Furthermore, a window structure semiconductor laser element as shown in FIG. 13 has also been proposed. In this window structure semiconductor laser element 60, a lower cladding layer 62, an active layer 63, an upper cladding layer 64, and a cap layer 65 are sequentially provided on a substrate 61. In this structure, in the vicinity of an end face from which laser light is to be emitted, a portion of the lower cladding layer 62, the active layer 63, and the upper cladding layer 64 are removed, and a p-block layer 66 and an n-block layer 67 are sequentially provided in place thereof. Both the block layers 66 and 67 are made of a transparent material which does not absorb light.
In the window structure semiconductor laser element 60 having such a structure, light is not absorbed by the end faces from which light is emitted. Thus, the occurrence of COD is prevented, and a high power laser emission is realized.
Furthermore, a broad area semiconductor laser element whose current confinement width is expanded for decreasing the entire density of light has been proposed.
Japanese Laid-Open Publication No. 11-68242 discloses a semiconductor laser element in which single mode waveguide areas are provided at both sides of a multimode waveguide area.
In the phase-locked array semiconductor laser element 50 shown in FIG. 12, a laser emission portion is structured in an array pattern. Because of such a structure, a resulting emission pattern is asymmetrical both in the vertical and horizontal directions, and therefore, the transmission area of the laser light is an elliptical shape. This laser element is not suitable for use as an optical transmitter attached to a ceiling, for example. Furthermore, it is necessary to synchronize the phases of the laser light between the active layer 53 and the wave-guiding layer 54b. Such a laser element is difficult to fabricate, and the production yield therefore decreases.
In the window structure semiconductor laser element 60 shown in FIG. 13, the occurrence of COD is prevented, whereby high power laser emission is realized. However, the injection of a large current into the active layer 63 may cause crystal defects, and may impede long-term, stable operation.
In the above-described broad area semiconductor laser element, an area in which a current is injected is relatively large. Therefore, the electrostatic capacitance of an entire laser element increases, and as a result, high speed modulation is not readily performed. Furthermore, the oscillation mode of this element is multimode, and the emission pattern is an elliptical shape is as in the phase-locked array semiconductor laser element. Such an element is not suitable for use as a light source for wireless transmission, for example.
Furthermore, the entirety of the semiconductor laser element disclosed in Japanese Laid-Open Publication No. 11-68242 is utilized as an active region. In such a laser element, COD occurs on the emission surface, and the output level is accordingly low. Furthermore, since the area in which the current is injected is large, the electrostatic capacitance is large. As a result, high speed modulation cannot be readily achieved.